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Arsenic and platinum deposition during cathodic hydrogenation
Scripta
METALLURGICA
V o l . 17, pp. 3 6 5 - 3 6 9 , 1 9 8 3 P r i n t e d in t h e U . S . A .
Pergamon
Press
Ltd
ARSENIC AND PLATINUM DEPOSITION DURING CATHODIC HYDROGENATION* M. B. Lewis and K. Farrell Metals and Ceramics Division, Oak Ridge National Laboratory Oak Ridge, Tennessee~ 37830 USA (Received
December
13,
1982)
The Problem The technique of cathodic charging is commonly used to introduce hydrogen into metals. Certain elements from Groups VA and VIA, notably As, P, S, Sb, Se, and Te, dissolved in the electrolyte are known to greatly promote the uptake of hydrogen at the cathode. In stainless steel, hydrogen concentrations of the order of 50 at. % are achieved using arsenic promoter in dilute sulphuric acid (1,2). The role of these promoters (or "poisons u as they are often called) is controversial. Suggested mechanisms include poisoning of the recombination rate of 2H+ + 2e-~ H2, increasing the surface adsorption of hydrogen, formation of hydrides of the promoter, altering the exchange current density, and reducing egress of hydrogen from the cathode. Whatever the mechanism, it requires accumulation of the promoter at the cathode surface. Yet there seems to be little or no documentation of such aggregation. This paper describes the finding of significant accumulation of promoter at the cathode. Aggregation of supposedly inert platinum anode material also occurred and was enhanced by the presence of the promoter. The Technique The specimens were type 310 austenitic stainless steel [21.1Ni, 23.4 Cr, 2 . 1 M n , 1.0 Si, 0.06 C (wt %)] annealed at 1325 K in helium. Disks of the alloy, 5 mm diam × 0.3 mm thick, were mechanically polished on Syntron lapping wheels to a 0.1~m diamond finish to provide a flat surface. Nickel wires were spot welded to the edges of the disks to suspend them as a cathode in the center of a cylindrical sheet anode of pure platinum. The charging conditions were typical of those used in hydrogen embrittlement studies of stainless steels, except that we used 99+% deuterated solutions so that we could employ nuclear microanalysls to demonstrate high deuterium concentrations in the cathode (2). The electrolyte was IN D2SO 4 held at 297 K. Arsenic concentrations of 2.5 and 250 mg per liter were investigated. These poisoned solutions were made by dissolving metallic arsenic powder in a little HCI + HNO 3 then adding a few drops of concentrated D2SO ~ and evaporating the solution until white SO 3 fumes prevailed; the concentrate was then made into a master solution with more D2SO 4 and D20, and was further diluted with D20 as required. During charging, the current density was controlled at IkA/m2. Charging times were in the range 1.2 × 10 2 to 8.6 × 10 4 s. The bath was stirred slowly with a plasticcoated magnetic paddle. Immediately following charging, each specimen was rinsed in water and ethanol then placed in a vacuum chamber for surface analysis. The surfaces of the charged specimens appeared to the naked eye to retain the same specular finish as the uncharged disks, except for specimens charged for periods much longer than one hour and for a lone specimen that was charged in boiling electrolyte at 373 K. These latter cases showed a slight surface tarnish. Our studies were focussed on the shorter charging times where films were not evident by visual inspection, but which were assumed to be present in less well-developed states. Direct, but delayed, evidence of such films was observed on specimens that were allowed to age in air at room temperature for periods of many months; they developed a golden patina. When this patina was removed by electropollshlng it did not return during subsequent aging. The analytical technique employed to determine surface contamination was Rutherford backscattering which measures the elastic scattering of a probing beam of particles incident on *Research sponsored by the Division of Materials Sciences, U. S. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corporation.
365 0036-9748/83/030365-05503.00/0
366
CATHODIC
HYDROGENATION
Vol.
17,
No.
3
the specimen surface. We used 2-MeV alpha particles from a Van de Graaff accelerator. These particles were recoiled from various atoms on and in the target, and were counted with respect to their recoil energy. The higher the atomic mass of the target atom, the more energetic the recoiling alpha particle, which allows identification of the mess of the atom. The yield of counts can be normalized and deconvoluted to extract a concentration profile in the surface film (3). Further details of these nuclear analytical techniques can be found in ref. 4. The method we used for the heavier elements is not very sensitive to elements lighter than nickel, so it would not detect small concentrations of oxygen and sulfur, the major cation components of the electrolyte. Results Examples of backscattering spectra for two cathodic charging conditions are shown in Fig. I. The channel numbers are proportional to the backscattered alpha e n e r g y . The high shoulders at the left (low energy) side of the figure are due to backscatterlng from the substrate cathode material. At higher energy (mass) levels, two distinct peaks are evident, labeled As and Pt. These two peaks correspond to amu ranges of 70 to 80 and 190 to 200 which are centered on As and Pt, as indicated by the positions of the arrows, but embrace Ga, Ge, As, Se, Br, and Os, It, Pt, Au, Hg, respectively. A check on these peaks using a 4 MeV alpha probe that increased the mess sensitivity to +-3 amu narrowed the contenders to Ge (72.6), As (75), Se (79), and Ir (192), Pt (195), Hg (200). Of these elements, only As and Pt are deliberate components in the charging system, so the peaks most probably represent As and Pt atoms. The slight shoulder on the left of the Pt peak at channel 275 may be an anomaly in the Pt profile, or it may arise from messes 106 (Pd) and/or 108 (Ag). Neither of these elements is known to be present in the system but they may be contaminants in the supposedly pure Pt anode. ~L
I Cr, Fe}
[
I F~
:
!
:
,.,.,L\I.I ~2
T - 297K
Z25
I
250
L
I
275 CHANNEL
"t'
300
Measurements were made of the amounts of Pt and As deposited on the cathode for various charging conditions. The results are listed in column 4 of Table I, Experiments without As in the electrolyte (charging conditions 1-2) were done for charging times of 6 × I02 s and 6 × I03 s. In these cases, Pt was found at the cathode in low concentrations, Increaslngwlth increasing time. The addition of As to the electrolyte at a concentration of 2.5 mg/l (charging conditions 3-6) introduced a relatlvely rapid and large build-up of As at the cathode, increasing with time; concurrently the deposition of Pt was considerably increased, by factors of I0 and 20 for charging times of 6 x 102 s and 6 x 10 3 s, respectlvely. Raising the nomlnal concentrations of As in the electrolyte to 250 mg/1 (conditions 7-8) did not slgnlflcantly change the deposition of Pt and As from that for the 2.5 mg/l conditions, at least for times >6 x 10 3 s. We see, then, that As is deposited faster than Pt, that the presence of As greatly increases the deposition of Pt, and that these effects are relatively insensitive to the As content of the electrolyte above 2.5 mg/l.
In one test we raised the temperature of the electrolyte to the boiling point, a procedure sometimes used in hydrogen embrittlement studies to encourage a more uniform distribution of hydrogen in the cathode. We found (condition 9 of Table I) that this higher temperature caused comparatively heavy deposition of As and Pt, especlally As. In fact, the deposit was sufficiently thick to permit measurement of a depth-concentratlon profile within the deposit. The results are shown in Fig. 2. The magnitude of the concentrations indicates that the As is probably metallic (atom ratio =I) in the deeper regions (i.e., the first deposits closest to the original cathode surface). They also imply that no other, unresolved ions are laid down in the initial deposit. The Pt exists in smeller concentration and presumably does not form a continuous metallic film. The sum of the concentrations of the two elements departs strongly from unity in the later layers. Here the balance of the composition is not known; a likely contributor is oxygen from the electrolyte or from exposure to air during transfer to the analysis chamber. Since the composition of the later layers of the film changes strongly, it becomes FIGURE 1 Backscatterlng spectra after two charging conditions, showing As and Pt peaks and, at left, the contribution from the stainless steel subs trate.
Vol.
17,
No.
3
CATHODIC
HYDROGENATION
367
TABLE 1 Amounts of As and Pt Deposited at 297 K, with Corresponding Deuterium Uptake in the Cathode Surface Layers
Charging Condition
Total Deposited Metal
Deuterium (Atomic
(mg/1)
(g/m 2)
Fraction)
6.0 x 10 2
0
~I x 10 -4 Pt
~1.2 x 10 -2
~6 x l 0 -3
6.0 x 10 3
0
1.3 x 10-3 Pt
0.15
0.15
1.2 x 10 2
2.5
2.6
3.1 x 10 -2 0.56
4 x l 0 -2
Charging Time
Arsenic in Solution
(s)
1.8
/
0.5
!
I
6.0 x I0 2
2.5
1.0 x 10 -3 Pt 1.8 x 10-3 As
0.12 0.56
0.2
6.0 x 10 3
2.5
2.7 I.I
3.2 3.4
0.8
8.3
2.5
9.3
x 10 W
x 10-3 Pt x 10 -3 As
8.6 x I0 ~
250
9.0 7.0
x 10- 2 Pt x 10-2 As
3.6 x 10 3 (T ffi 373 K)
250
2.0 x I0-i Pt 8.1 x I0-I As
I
['"
I
I
ORIGINAL CATHODE SURFACE
I;E -
8 T = 373K t = 3.6 x | 0 3 |
As • 250 rag/!
I
x i0-2
4.6 9.3
As
50
x 10 -2 Pt As
8.0
250
~_0.05
o
x 10 -2 Pt x 10 -2 As
6.0 x I0 3
0.!
o c~
x 10 -4 Pt 10-3 As
x
Monolayers
I I ~ 15o DEPTH IN AI LAYER (nm)
I 200
FIGURE 2 Depth-concentratlon profiles of As and Pt in a thick layer deposited during hot charging at 373 K.
II 25 0.53 2.9 11 22
0.7
0.7
0.7
---
difficult to assign a density to the film, which introduces greater uncertainties into the estimated depths at the shallower depths. Figure 2 and Table I are largely self-consistent if we accept that Pt is deposited at an approximately constant rate throughout the duration of charging, whereas the As is initially deposited at a much higher rate forming a continuous As layer on the cathode surface in the first few minutes. Following this, the As deposition rate decreases for the remaining greater part of the electrocharging period so that the composition of the film becomes richer in Pt. This decline in the deposition rate of As is not due to depletion of As from the electrolyte since the amount removed is small compared to that available. For example, each electrolysis was done in I00 ml of electrolyte which, at the lower As level, contained 0.25 mg of As. This is sufficient to cover more than 60 specimens with a thickness of 8 x 10 -2 g/m 2, as measured for a charging time of 8.3 x 10 % s. Relationship
to Deuterium Absorption
The concentrations of deuterium absorbed in the immediate subsurface layers of the cathode at cessation of charging are given in column 6 of Table I. These concentrations increase with charging time, more rapidly when As is present in the electrolyte, in
568
CATHODIC HYDROGENATION
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17, No. 5
which cases they soon reach a saturation level of 0.7 to 0.8 atomic fraction. Without As, the saturation level was not reached at 6 × 10 3 s. This dependence of deuterium concentration on time is contrary to expectations. Since the applied electrical conditions were held constant in this work, the chemical potential (fugaclty) of deuterium at the cathode should be constant and should be achieved instantly. The corresponding "equilibrium" concentrations of deuterium in the surface layers should follow the same pattern unless something happens to change the deuterium potential or the surface activity. Clearly, something does happen because the deuterium concentrations do not reach their ultimate saturation value iwmediately, but require a sluggish transition period. And time is not the primary determinant, as seen by comparing charging condition 2 with condition 4, where different times give roughly equal deuterium concentrations, and comparison of charging condition 2 with condition 5, where similar times cause dlffereDt deuterium levels. Rather, the controlling factor seems to be the presence of deposited metal on the cathode. We have suggested in an earlier paper (2) written before our discovery of these deposited metal films, that during the transition period required to reach the deuterium saturation value, some "conditioning" of the cathode surface was occurring. We now propose that such conditioning involves the deposition of the metal film on the original cathode surface. This film contains As and/or Pt. Significance
of Degree of Surface Coverage
It follows from the above deductions that the degree of coverage of the cathode surface by the deposited metal on an atomic scale might be an important factor in deuterium absorption. To assess this we have translated the thicknesses of the films into atomic layers, as listed in column 5 of Table I. These monolayer values are very approximate and are obtained by dividing the amount deposited (in g/m 2) by the product of the metal's density and its lattice spacing. In this way the As and Pt deposits are artificially treated as separate layers, which may not be too unreasonable in view of the findings on the hot-charged specimen. We note that in the absence of As, the amounts of Pt comprise only fractions of a monolayer, and the deuterium concentrations in the cathode subsurface layers are correspondingly low. With As in the electrolyte, almost a monolayer of As is deposited on the cathode even In the shortest charging time (condition 3, Table I). Although the corresponding deuterium concentration in the cathode is boosted it remains well below the eventual saturation level. However, the Pt coverage is also increased by the presence of As, and the deuterium concentration is raised proportionally. Deuterium saturation at 0.7 to 0.8 atomic fraction is reached when the surface coverage by Pt and As reaches or exceeds about a monolayer. This is shown graphically in Flg. 3. Discussion
i0-! ELECTROCHARGED 5tO STAINLESS STEEL 297 K
,o'
qo"
tOo
i~10-3
iO-4
10-3
I0-'
tO-z
40"
tO°
OEUTERIUM (otomi¢ fre¢tlon)
FIGURE 3 Dependence of deuterium uptake on surface coverage by deposited As and Pt. Solid points are data taken with As in the electrolyte; open circles represent no As.
It seems that the deuterium concentration in the cathode subsurface layers is a sensitive function of the degree of Pt coverage. Coverage by As could also be an important factor. Unfortunately, we could not determine the effects of As deposition alone. These few experiments suggest that perhaps the prime function of As in promoting deuterium uptake by the cathode is to accelerate the deposition of anode material which is the real (or the stronger) promoter. From the data we can draw these tentative conclusions: (I) the deuterium concentration in the cathode surface layers does not adjust i~mediately to satisfy the high deuterium fugacity, (2) the level of deuterium uptake increases concurrently with the build-up of a layer of promoter (As) and/or anode material (Pt) at the cathode, and (3) the deuterium concentration appears to reach saturation when the amount of As and Pt deposited reaches or exceeds about a monolayer thickness.
Vol.
17,
No.
3
CATHODIC
HYDROGENATION
369
An explanation consistent with these observations is that the original cathode surface is covered with an imperceptlble'atmospherlc contamination film which resists adsorption, penetration and/or retention of deuterium. Laying a fresh metal layer over the contamination film during cathodic charging overcomes the film's resistance to deuterium absorption, possibly by reducing the 2H+ + 2e- + H 2 reaction. This is analogous to experience with hydrogen absorption from the gaseous phase at elevated temperatures where it is found that oxide films on fcc alloys reduce the permeation of hydrogen isotopes by orders of magnitude (6,7), but a coat of palladium on top of the oxide scale overcomes the inhibiting effects of the oxide (8). Evidently, the inhibiting effect of the oxide film is associated with a surface reaction, not with reduced rate of diffusion of hydrogen through the oxide scale. Similar considerations might apply to cathodic hydrogenation. Diffuslonal effects might become important, however, in thick surface coatings if hydrogen diffusion through the coatings is significantly less than that through the substrate. Relevance for Some H~dro$en Embrittlement Studies While these findings are especially relevant to hydrogen absorption in metals during electrocharging, they may be of further importance to some hydrogen embrittlement studies. There seems to be an increasing trend towards doing hydrogen embrittlement tests under dynamic char~ing conditions in poisoned electrolytes. The assumption is that the embrittlement displayed in such tests is due solely to hydrogen. Possible effects of the poison and/or anodic materials on altcrin~ fracture ~rocesses and exacerbating embrittlement are usually overlooked. Yet the elements most commonly used as poisons are known or suspected embrittllng agents in their own right, especially in the phenomenon of temper embrlttlement (9), where the tests are made at similar temperatures to those for hydrogen embrittlement. The present findings indicate that care should be taken to ascertain whether elements other than hydrogen are deposited at the specimen surface and at exposed cracks opened during the test, and are contributing to the embrittlement. Conclusions These experiments show that arsenic promoter and supposedly inert platinum anode material are co-deposited with hydrogen on the cathode surface during cathodic hydrogenation. The arsenic is initially laid down faster than the platinum, and it accelerates the deposition of platinum. Hydrogen (deuterium) uptake by the cathode appears to increase to a high saturation level as the amount of metal deposited reaches the equivalent of a monolayer or more of surface coverage. It is cautioned that the presence of these deposits should be considered in hydrogen embrlttlement tests made under dynamic cathodic electrolysis. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
A. Atrens, J. J. Bellina, N. F. Fiore, and R. J. Coyle, pp. 54-69 in The Metal Science of Stainless Steels, eds., E. W. Collings and H. W. King, Met. Soc. AIME (1979). K. Farrell and M. B. Lewis, Scrlpta Met. 15, 661 (1981). M. B. Lewis, Nucl. Inst. Methods 190, 605 (1981). W. K. Chu, J. W. Mayer, and M. A. Nicolet in Backscatterin~ Spectrometry, Academic Press, New York (1978). M. B. Lewis and K. Farrell, Appl. Phys. Lett. 36(10), 819 (1980). R. A. Strehlow and H. C. Savage, Nucl. Technol. 22, 127 (1974). J. T. Bell, J. D. Redman, and F. J. Smith, p. 3 in Proceedings of 24th Conference on Remote Systems Technology (1976). W. A. Swanslger and R. Bastasz, J. Nucl. Mater. 85&86, 335 (1979). W. Steven and K. BalaJiva, J. Iron and Steel Inst. 193, 141 (1959).
METALLURGICA
V o l . 17, pp. 3 6 5 - 3 6 9 , 1 9 8 3 P r i n t e d in t h e U . S . A .
Pergamon
Press
Ltd
ARSENIC AND PLATINUM DEPOSITION DURING CATHODIC HYDROGENATION* M. B. Lewis and K. Farrell Metals and Ceramics Division, Oak Ridge National Laboratory Oak Ridge, Tennessee~ 37830 USA (Received
December
13,
1982)
The Problem The technique of cathodic charging is commonly used to introduce hydrogen into metals. Certain elements from Groups VA and VIA, notably As, P, S, Sb, Se, and Te, dissolved in the electrolyte are known to greatly promote the uptake of hydrogen at the cathode. In stainless steel, hydrogen concentrations of the order of 50 at. % are achieved using arsenic promoter in dilute sulphuric acid (1,2). The role of these promoters (or "poisons u as they are often called) is controversial. Suggested mechanisms include poisoning of the recombination rate of 2H+ + 2e-~ H2, increasing the surface adsorption of hydrogen, formation of hydrides of the promoter, altering the exchange current density, and reducing egress of hydrogen from the cathode. Whatever the mechanism, it requires accumulation of the promoter at the cathode surface. Yet there seems to be little or no documentation of such aggregation. This paper describes the finding of significant accumulation of promoter at the cathode. Aggregation of supposedly inert platinum anode material also occurred and was enhanced by the presence of the promoter. The Technique The specimens were type 310 austenitic stainless steel [21.1Ni, 23.4 Cr, 2 . 1 M n , 1.0 Si, 0.06 C (wt %)] annealed at 1325 K in helium. Disks of the alloy, 5 mm diam × 0.3 mm thick, were mechanically polished on Syntron lapping wheels to a 0.1~m diamond finish to provide a flat surface. Nickel wires were spot welded to the edges of the disks to suspend them as a cathode in the center of a cylindrical sheet anode of pure platinum. The charging conditions were typical of those used in hydrogen embrittlement studies of stainless steels, except that we used 99+% deuterated solutions so that we could employ nuclear microanalysls to demonstrate high deuterium concentrations in the cathode (2). The electrolyte was IN D2SO 4 held at 297 K. Arsenic concentrations of 2.5 and 250 mg per liter were investigated. These poisoned solutions were made by dissolving metallic arsenic powder in a little HCI + HNO 3 then adding a few drops of concentrated D2SO ~ and evaporating the solution until white SO 3 fumes prevailed; the concentrate was then made into a master solution with more D2SO 4 and D20, and was further diluted with D20 as required. During charging, the current density was controlled at IkA/m2. Charging times were in the range 1.2 × 10 2 to 8.6 × 10 4 s. The bath was stirred slowly with a plasticcoated magnetic paddle. Immediately following charging, each specimen was rinsed in water and ethanol then placed in a vacuum chamber for surface analysis. The surfaces of the charged specimens appeared to the naked eye to retain the same specular finish as the uncharged disks, except for specimens charged for periods much longer than one hour and for a lone specimen that was charged in boiling electrolyte at 373 K. These latter cases showed a slight surface tarnish. Our studies were focussed on the shorter charging times where films were not evident by visual inspection, but which were assumed to be present in less well-developed states. Direct, but delayed, evidence of such films was observed on specimens that were allowed to age in air at room temperature for periods of many months; they developed a golden patina. When this patina was removed by electropollshlng it did not return during subsequent aging. The analytical technique employed to determine surface contamination was Rutherford backscattering which measures the elastic scattering of a probing beam of particles incident on *Research sponsored by the Division of Materials Sciences, U. S. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corporation.
365 0036-9748/83/030365-05503.00/0
366
CATHODIC
HYDROGENATION
Vol.
17,
No.
3
the specimen surface. We used 2-MeV alpha particles from a Van de Graaff accelerator. These particles were recoiled from various atoms on and in the target, and were counted with respect to their recoil energy. The higher the atomic mass of the target atom, the more energetic the recoiling alpha particle, which allows identification of the mess of the atom. The yield of counts can be normalized and deconvoluted to extract a concentration profile in the surface film (3). Further details of these nuclear analytical techniques can be found in ref. 4. The method we used for the heavier elements is not very sensitive to elements lighter than nickel, so it would not detect small concentrations of oxygen and sulfur, the major cation components of the electrolyte. Results Examples of backscattering spectra for two cathodic charging conditions are shown in Fig. I. The channel numbers are proportional to the backscattered alpha e n e r g y . The high shoulders at the left (low energy) side of the figure are due to backscatterlng from the substrate cathode material. At higher energy (mass) levels, two distinct peaks are evident, labeled As and Pt. These two peaks correspond to amu ranges of 70 to 80 and 190 to 200 which are centered on As and Pt, as indicated by the positions of the arrows, but embrace Ga, Ge, As, Se, Br, and Os, It, Pt, Au, Hg, respectively. A check on these peaks using a 4 MeV alpha probe that increased the mess sensitivity to +-3 amu narrowed the contenders to Ge (72.6), As (75), Se (79), and Ir (192), Pt (195), Hg (200). Of these elements, only As and Pt are deliberate components in the charging system, so the peaks most probably represent As and Pt atoms. The slight shoulder on the left of the Pt peak at channel 275 may be an anomaly in the Pt profile, or it may arise from messes 106 (Pd) and/or 108 (Ag). Neither of these elements is known to be present in the system but they may be contaminants in the supposedly pure Pt anode. ~L
I Cr, Fe}
[
I F~
:
!
:
,.,.,L\I.I ~2
T - 297K
Z25
I
250
L
I
275 CHANNEL
"t'
300
Measurements were made of the amounts of Pt and As deposited on the cathode for various charging conditions. The results are listed in column 4 of Table I, Experiments without As in the electrolyte (charging conditions 1-2) were done for charging times of 6 × I02 s and 6 × I03 s. In these cases, Pt was found at the cathode in low concentrations, Increaslngwlth increasing time. The addition of As to the electrolyte at a concentration of 2.5 mg/l (charging conditions 3-6) introduced a relatlvely rapid and large build-up of As at the cathode, increasing with time; concurrently the deposition of Pt was considerably increased, by factors of I0 and 20 for charging times of 6 x 102 s and 6 x 10 3 s, respectlvely. Raising the nomlnal concentrations of As in the electrolyte to 250 mg/1 (conditions 7-8) did not slgnlflcantly change the deposition of Pt and As from that for the 2.5 mg/l conditions, at least for times >6 x 10 3 s. We see, then, that As is deposited faster than Pt, that the presence of As greatly increases the deposition of Pt, and that these effects are relatively insensitive to the As content of the electrolyte above 2.5 mg/l.
In one test we raised the temperature of the electrolyte to the boiling point, a procedure sometimes used in hydrogen embrittlement studies to encourage a more uniform distribution of hydrogen in the cathode. We found (condition 9 of Table I) that this higher temperature caused comparatively heavy deposition of As and Pt, especlally As. In fact, the deposit was sufficiently thick to permit measurement of a depth-concentratlon profile within the deposit. The results are shown in Fig. 2. The magnitude of the concentrations indicates that the As is probably metallic (atom ratio =I) in the deeper regions (i.e., the first deposits closest to the original cathode surface). They also imply that no other, unresolved ions are laid down in the initial deposit. The Pt exists in smeller concentration and presumably does not form a continuous metallic film. The sum of the concentrations of the two elements departs strongly from unity in the later layers. Here the balance of the composition is not known; a likely contributor is oxygen from the electrolyte or from exposure to air during transfer to the analysis chamber. Since the composition of the later layers of the film changes strongly, it becomes FIGURE 1 Backscatterlng spectra after two charging conditions, showing As and Pt peaks and, at left, the contribution from the stainless steel subs trate.
Vol.
17,
No.
3
CATHODIC
HYDROGENATION
367
TABLE 1 Amounts of As and Pt Deposited at 297 K, with Corresponding Deuterium Uptake in the Cathode Surface Layers
Charging Condition
Total Deposited Metal
Deuterium (Atomic
(mg/1)
(g/m 2)
Fraction)
6.0 x 10 2
0
~I x 10 -4 Pt
~1.2 x 10 -2
~6 x l 0 -3
6.0 x 10 3
0
1.3 x 10-3 Pt
0.15
0.15
1.2 x 10 2
2.5
2.6
3.1 x 10 -2 0.56
4 x l 0 -2
Charging Time
Arsenic in Solution
(s)
1.8
/
0.5
!
I
6.0 x I0 2
2.5
1.0 x 10 -3 Pt 1.8 x 10-3 As
0.12 0.56
0.2
6.0 x 10 3
2.5
2.7 I.I
3.2 3.4
0.8
8.3
2.5
9.3
x 10 W
x 10-3 Pt x 10 -3 As
8.6 x I0 ~
250
9.0 7.0
x 10- 2 Pt x 10-2 As
3.6 x 10 3 (T ffi 373 K)
250
2.0 x I0-i Pt 8.1 x I0-I As
I
['"
I
I
ORIGINAL CATHODE SURFACE
I;E -
8 T = 373K t = 3.6 x | 0 3 |
As • 250 rag/!
I
x i0-2
4.6 9.3
As
50
x 10 -2 Pt As
8.0
250
~_0.05
o
x 10 -2 Pt x 10 -2 As
6.0 x I0 3
0.!
o c~
x 10 -4 Pt 10-3 As
x
Monolayers
I I ~ 15o DEPTH IN AI LAYER (nm)
I 200
FIGURE 2 Depth-concentratlon profiles of As and Pt in a thick layer deposited during hot charging at 373 K.
II 25 0.53 2.9 11 22
0.7
0.7
0.7
---
difficult to assign a density to the film, which introduces greater uncertainties into the estimated depths at the shallower depths. Figure 2 and Table I are largely self-consistent if we accept that Pt is deposited at an approximately constant rate throughout the duration of charging, whereas the As is initially deposited at a much higher rate forming a continuous As layer on the cathode surface in the first few minutes. Following this, the As deposition rate decreases for the remaining greater part of the electrocharging period so that the composition of the film becomes richer in Pt. This decline in the deposition rate of As is not due to depletion of As from the electrolyte since the amount removed is small compared to that available. For example, each electrolysis was done in I00 ml of electrolyte which, at the lower As level, contained 0.25 mg of As. This is sufficient to cover more than 60 specimens with a thickness of 8 x 10 -2 g/m 2, as measured for a charging time of 8.3 x 10 % s. Relationship
to Deuterium Absorption
The concentrations of deuterium absorbed in the immediate subsurface layers of the cathode at cessation of charging are given in column 6 of Table I. These concentrations increase with charging time, more rapidly when As is present in the electrolyte, in
568
CATHODIC HYDROGENATION
Vol.
17, No. 5
which cases they soon reach a saturation level of 0.7 to 0.8 atomic fraction. Without As, the saturation level was not reached at 6 × 10 3 s. This dependence of deuterium concentration on time is contrary to expectations. Since the applied electrical conditions were held constant in this work, the chemical potential (fugaclty) of deuterium at the cathode should be constant and should be achieved instantly. The corresponding "equilibrium" concentrations of deuterium in the surface layers should follow the same pattern unless something happens to change the deuterium potential or the surface activity. Clearly, something does happen because the deuterium concentrations do not reach their ultimate saturation value iwmediately, but require a sluggish transition period. And time is not the primary determinant, as seen by comparing charging condition 2 with condition 4, where different times give roughly equal deuterium concentrations, and comparison of charging condition 2 with condition 5, where similar times cause dlffereDt deuterium levels. Rather, the controlling factor seems to be the presence of deposited metal on the cathode. We have suggested in an earlier paper (2) written before our discovery of these deposited metal films, that during the transition period required to reach the deuterium saturation value, some "conditioning" of the cathode surface was occurring. We now propose that such conditioning involves the deposition of the metal film on the original cathode surface. This film contains As and/or Pt. Significance
of Degree of Surface Coverage
It follows from the above deductions that the degree of coverage of the cathode surface by the deposited metal on an atomic scale might be an important factor in deuterium absorption. To assess this we have translated the thicknesses of the films into atomic layers, as listed in column 5 of Table I. These monolayer values are very approximate and are obtained by dividing the amount deposited (in g/m 2) by the product of the metal's density and its lattice spacing. In this way the As and Pt deposits are artificially treated as separate layers, which may not be too unreasonable in view of the findings on the hot-charged specimen. We note that in the absence of As, the amounts of Pt comprise only fractions of a monolayer, and the deuterium concentrations in the cathode subsurface layers are correspondingly low. With As in the electrolyte, almost a monolayer of As is deposited on the cathode even In the shortest charging time (condition 3, Table I). Although the corresponding deuterium concentration in the cathode is boosted it remains well below the eventual saturation level. However, the Pt coverage is also increased by the presence of As, and the deuterium concentration is raised proportionally. Deuterium saturation at 0.7 to 0.8 atomic fraction is reached when the surface coverage by Pt and As reaches or exceeds about a monolayer. This is shown graphically in Flg. 3. Discussion
i0-! ELECTROCHARGED 5tO STAINLESS STEEL 297 K
,o'
qo"
tOo
i~10-3
iO-4
10-3
I0-'
tO-z
40"
tO°
OEUTERIUM (otomi¢ fre¢tlon)
FIGURE 3 Dependence of deuterium uptake on surface coverage by deposited As and Pt. Solid points are data taken with As in the electrolyte; open circles represent no As.
It seems that the deuterium concentration in the cathode subsurface layers is a sensitive function of the degree of Pt coverage. Coverage by As could also be an important factor. Unfortunately, we could not determine the effects of As deposition alone. These few experiments suggest that perhaps the prime function of As in promoting deuterium uptake by the cathode is to accelerate the deposition of anode material which is the real (or the stronger) promoter. From the data we can draw these tentative conclusions: (I) the deuterium concentration in the cathode surface layers does not adjust i~mediately to satisfy the high deuterium fugacity, (2) the level of deuterium uptake increases concurrently with the build-up of a layer of promoter (As) and/or anode material (Pt) at the cathode, and (3) the deuterium concentration appears to reach saturation when the amount of As and Pt deposited reaches or exceeds about a monolayer thickness.
Vol.
17,
No.
3
CATHODIC
HYDROGENATION
369
An explanation consistent with these observations is that the original cathode surface is covered with an imperceptlble'atmospherlc contamination film which resists adsorption, penetration and/or retention of deuterium. Laying a fresh metal layer over the contamination film during cathodic charging overcomes the film's resistance to deuterium absorption, possibly by reducing the 2H+ + 2e- + H 2 reaction. This is analogous to experience with hydrogen absorption from the gaseous phase at elevated temperatures where it is found that oxide films on fcc alloys reduce the permeation of hydrogen isotopes by orders of magnitude (6,7), but a coat of palladium on top of the oxide scale overcomes the inhibiting effects of the oxide (8). Evidently, the inhibiting effect of the oxide film is associated with a surface reaction, not with reduced rate of diffusion of hydrogen through the oxide scale. Similar considerations might apply to cathodic hydrogenation. Diffuslonal effects might become important, however, in thick surface coatings if hydrogen diffusion through the coatings is significantly less than that through the substrate. Relevance for Some H~dro$en Embrittlement Studies While these findings are especially relevant to hydrogen absorption in metals during electrocharging, they may be of further importance to some hydrogen embrittlement studies. There seems to be an increasing trend towards doing hydrogen embrittlement tests under dynamic char~ing conditions in poisoned electrolytes. The assumption is that the embrittlement displayed in such tests is due solely to hydrogen. Possible effects of the poison and/or anodic materials on altcrin~ fracture ~rocesses and exacerbating embrittlement are usually overlooked. Yet the elements most commonly used as poisons are known or suspected embrittllng agents in their own right, especially in the phenomenon of temper embrlttlement (9), where the tests are made at similar temperatures to those for hydrogen embrittlement. The present findings indicate that care should be taken to ascertain whether elements other than hydrogen are deposited at the specimen surface and at exposed cracks opened during the test, and are contributing to the embrittlement. Conclusions These experiments show that arsenic promoter and supposedly inert platinum anode material are co-deposited with hydrogen on the cathode surface during cathodic hydrogenation. The arsenic is initially laid down faster than the platinum, and it accelerates the deposition of platinum. Hydrogen (deuterium) uptake by the cathode appears to increase to a high saturation level as the amount of metal deposited reaches the equivalent of a monolayer or more of surface coverage. It is cautioned that the presence of these deposits should be considered in hydrogen embrlttlement tests made under dynamic cathodic electrolysis. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
A. Atrens, J. J. Bellina, N. F. Fiore, and R. J. Coyle, pp. 54-69 in The Metal Science of Stainless Steels, eds., E. W. Collings and H. W. King, Met. Soc. AIME (1979). K. Farrell and M. B. Lewis, Scrlpta Met. 15, 661 (1981). M. B. Lewis, Nucl. Inst. Methods 190, 605 (1981). W. K. Chu, J. W. Mayer, and M. A. Nicolet in Backscatterin~ Spectrometry, Academic Press, New York (1978). M. B. Lewis and K. Farrell, Appl. Phys. Lett. 36(10), 819 (1980). R. A. Strehlow and H. C. Savage, Nucl. Technol. 22, 127 (1974). J. T. Bell, J. D. Redman, and F. J. Smith, p. 3 in Proceedings of 24th Conference on Remote Systems Technology (1976). W. A. Swanslger and R. Bastasz, J. Nucl. Mater. 85&86, 335 (1979). W. Steven and K. BalaJiva, J. Iron and Steel Inst. 193, 141 (1959).